1. Field of the Invention
The present invention relates to code excited linear prediction of speech signals and more particularly to a fixed code book with an embedded adaptive code book.
2. Background of the Invention
Code excited linear prediction coders are used to generate human speech signals in bandwidth limited environments, using what is sometimes referred to as “analysis by synthesis.” In particular, a code excited linear prediction coder uses a fixed code book excitation signal that relies on stored excitation vector parameter codes that are selected from a code book, and adaptive code book excitation vectors parameters that are representative of long-term processing. The adaptive code book typically contains candidate vectors constructed from past synthesized excitation which are available in the decoder. The number of the candidates in the adaptive code book is usually much smaller than the number in the fixed code book, and the adaptive code book is continually updated based on past excitation vector parameters.
Although such fixed and adaptive excitation code books provide acceptable speech encoding in most cases, when a significant difference in the spectrum or pitch lag of the voice signal occurs, long term processing through the use of adaptive code book entries can occasionally fail. Although a switch to traditional long term processing can be made when long term processing in a CELP coder fails, the switching can cause the algorithm to be less robust to bit errors, such that noticeable speech errors are generated. When the pitch lag is coded in a differential way to save the bit rate, even the traditional long term prediction could fail.